Presidential elections are determined by the electoral college.
Determining how to unseat Trump is going to involve figuring out how to win the swing states.
It's not exclusively the GOP. Single payer is a more liberal version of the Public Option that was too left-wing for Democrats when they had control of Congress and the White House a decade ago.
As for why people don't support it, there are significant tradeoffs. Taking out the profit motive could disincentivize innovation. Someone is going to have to make the difficult choices of when something is too expensive to be worth the cost.
The biggest problem may be that a significant chunk of voters are happy with employer-based health insurance (a legacy on wage caps during World War 2.) This is obviously a system that doesn't work for everyone, but does mean there are stakeholders afraid of losing the current status quo.
Trump is making non-trivial improvements among African American voters.
https://www.vox.com/policy-and-polit...erican-polling
And she lost to a guy who was caught on tape grabbing women by the pussy.
This raises an interesting question: How often do we actually get an exciting candidate? How do we gauge that?
One factor is that different voters will find different things exciting.
Even the candidates we might agree are generally considered exciting (Bill Clinton, Barack Obama) ran in historically favorable general election environments.